High molecular weight linear thermoplastic polyesters, such as poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) are well known as film and fiber-formers and they are provided by methods outlined in Whinfield et al. U.S. Pat. No. 2,465,319 and Pengilly, U.S. Pat. No. 3,047,539, and elsewhere.
Polyester resins, such as poly(alkylene terephthalates, isophthalates and mixed iso- and terephthalates), particularly poly(1,4-butylene terephthalate), also provide excellent molding compositions because they can be fabricated with moderate stock temperatures, low mold temperatures and rapid cycle times. Because of their unusual combination of properties, these resins are superior in chemical resistance, thermal stability and product appearance (they have a smooth, glossy finish). Such resins also have superior strength, stiffness, low friction and wear properties and good resistance to brittle fracture. The polyester resins can also be provided in reinforced embodiments.
Solid polyester resins of the type mentioned have found extensive use in various forms of thermoplastic structures. Many different articles of manufacture are produced from polyester compositions by molding and shaping at higher temperatures because they remain relatively rigid at ordinary room temperature. Polyester fibers are also useful in woven articles and films therefrom are both useful as mechanical supports and as barriers and also they have useful protective coating applications. Although the thermoplastic molding and coating compositions have the outstanding properties mentioned above, they unfortunately are subject to polymeric degradation during processing due to thermal and oxidation instability of the polyester component. Deterioration caused by heat and/or oxygen will lead to loss of electrical properties and/or to discoloration, embrittlement, and especially loss of physical properties such as tensile strength and impact strength. Additionally, discoloration, which is undesirable in some uses, may occur if the polyester resin composition is not stabilized with proper amounts and kinds of stabilizers.
Poly(alkylene terephthalate) and particularly poly(1,4-butylene terephthalate) undergo a gradual degradation due to thermal decomposition during processing, e.g., extrusion, compounding and molding. This degradation is due to thermal scission, and is accompanied by a decrease in melt viscosity and a deterioration of the polyester's physical and mechanical properties. Thermal scission can be observed experimentally by a drop in melt viscosity in well-known instruments, such as an extrusion plastometer melt indexer (e.g., ASTM D-1238) in which the molten material is forced through a standard orifice and the rate of extrusion (e.g., unit weight recovered in a fixed time) is measured and recorded. Typically, degradation in chain length due to thermal scission is reflected in a higher melt index. If the data are numerically expressed, for example, a decrease in the logarithm of the melt viscosity per minute time [.DELTA.(log.eta.)/.DELTA.t], the so-called degradation constant is provided and this can be compared with controls. Typical values for the degradation constant of poly(1,4-butylene terephthalate) are 6 .times. 10.sup.-.sup.3 min.sup.-.sup.1 (250.degree.C.); and 12 .times. 10.sup.-.sup.3 min.sup.-.sup.1 (260.degree.C.).
Typical methods will be described hereinafter.
Commonly employed stabilizers for polyester resin systems such as the hindered phenols and phosphite chelators, improve the long-term oxidative stability of the compositions, but have no effect whatsoever on the melt stability as measured by degradation constant.
Surprisingly, however, it has now been found that small amounts of polybutadienes, containing high amounts, e.g., &gt; 85 mol % of the units of vinyl unsaturation vastly improve the melt stability of polyester resins.
By way of illustration, poly(1,4-butylene terephthalate) containing 1 part of hydroxy-terminated polybutadiene had degradation constants of -3.3 .times. 10.sup.-.sup.3 min.sup.-.sup.1 (250.degree.C.); -4.5 .times. 10.sup.-.sup.3 min.sup.-.sup.1 and -5.6 .times. 10.sup.-.sup.3 min.sup.-.sup.1 (260.degree.C., duplicate tests). The minus sign indicates a most desirable increase in melt viscosity. Moreover, with 0.2 parts of a phenolic antioxidant included, the degradation constants were also excellent: 0.28 .times. 10.sup.-.sup.3 min.sup.-.sup.1 ; 1.33 .times. 10.sup.-.sup.3 min.sup.-.sup.1 ; and -1.14 .times. 10.sup.-.sup.3 min.sup.-.sup.1 (250.degree.C., triplicate determinations).
In addition to stabilizing the melt viscosity, the high vinyl polybutadiene also retarded discoloration of the polyester composition due to thermal oxidation of the melt.
It is, accordingly, a principal object of the invention to provide stabilized compositions of normally unstable polyester resins, which are normally rigid at room temperature. It is another object of the invention to provide such stabilized compositions by incorporating in a normally unstable high molecular weight polyester resin a minor portion of a stabilizer which comprises a high vinyl polybutadiene and, optionally, a hindered phenolic antioxidant and/or an organic phosphite costabilizer. It is a further object to provide reinforced embodiments of such stabilized compositions.